Apparatus for preventing back flash in gas lines



Oct. 1936- P. s. WILLIAMS ET AL 2,058,550

APPARATUS FOR 'PREVENTING' BACK FLASH IN GAS LINES I Filed April 15, 1952 2 Sheec .s-She,t 1 v N ENTOR3. bl/lp .S.v i/fdms and ATTORNEY Oct. 27, 1936. P. s. WILLIAMS ET AL I 2 4 APPARATUS FORTREVENTING BACK FLASH IN GAS LINES Filed April 15, 1932 2 Sheets-Sheet 2 8 INVENTOR5 fiW/p S. M/flam: 0/74 ATTORNEY Patented Oct. 27, 1936 UNiTEfi STATES OFFICE APPARATUS FOR PREVENTING BACK FLASH IN GAS LINES Application April 15, 1932, Serial No. 605,544

6 Claims.

This invention relates to a method and apparatus for arresting flame travel through a gaseous, combustion supporting mixture.

In the handling of gaseous mixtures during sampling, testing, transportation, storage or utilization in a conduit or container system, a mixture capable of sustaining and propagating a flame may be knowingly or unknowingly created. While a flame propagating mixture may accidently become ignited in a portion of the system where it will not do any appreciable harm, as in a pipe line or conduit, there is always a possibility that the flame will reach another part of the system, as a container or tank, and great harm to life and property will result. Flame arrestors have been devised and employed in an attempt to confine a flame to a relatively small portion of the system containing the gaseous mixture and so prevent the flame from reaching all parts of the system. The present invention is concerned with a method of arresting flame travel and an apparatus which has been successfully used in preventing flame travel beyond an arrestor in the diflicult and fast burning hydrogenair mixtures.

Probably the simplest form of arrestor developed against flame travel in a gaseous mixture is a fine mesh screen. However, such an arrestor is subject to numerous disadvantages including stoppage by dust and other foreign solid particles and loss of strength by corrosion. Also since its effectiveness depends on the size of the openings, practical size limitations are imposed on the utilization of such an arrestor against flames in hydrogen-air, acetylene-air and like mixtures. This limitation was brought out in a recent study of the size of aperture necessary to prevent flame propagation through hydrogen-air and acetylene: air mixtures from electrical apparatus to the end that flame proof electrical apparatus could be designed. It is there stated (see Flame Proof Electrical Apparatus, by Rainford and Statham, vol 10, No. 12, pages 504-520) that the permissible safe gapfor an inch flange was 0.0015 inch. The authors state this is no gap but the space present between the usual flanges when drawn up and that this gap is subject to so great a variation that, from a practical standpoint, electrical apparatus, proof against hydrogen-air mixtures cannot at present be designed where dependence is placed on a small gap as the flame arrestor.

The practical importance of the present invention may be further appreciated when it is pointed out that heretofore an arrestor eifective against hydrogen-air and acetylene-air mixtures has not been available. Any apparatus depending upon a flame arrestor has, therefore, been limited in scope of application and safeness in operation.

This invention enables flame propagation to be stopped in any mixture in any proportion of any gaseous combustible constituent or constituents (including hydrogen, acetylene and constituents approaching these in flame propagating rates) with air. Generically, this invention has for its object the providing of a method of arresting flame propagation in gaseous combustible mixtures with air and apparatus for practising the invention.

The invention is also concerned with, and has for other objects, a method of determining the combustible content of a gas mixture and the control of this determination to eliminate the danger of a flame being propagated from the apparatus used to determine the combustible content of a quantity of the gas mixture.

The invention possesses other objects and features of advantage, some of which, with the foregoing, will appear at length in the following specification wherein we have disclosed the preferred method and apparatus for practising our invention.

In the drawings, accompanying and forming a part of this specification:

Figure 1 is a diagrammatic showing of equipment employed in practising the invention.

Figure 2 is a diagram of the Wheatstone bridge circuit employed.

Figure 3 is a section illustrating the construction of a gas reaction cell with which our flame arrestor is employed advantageously.

Figures 4 to 15 inclusive are other views, largely schematic in nature, illustrating the construction and arrangement of successful flame arrestors. Figure 8 is a section on the line 88 of Figures '7 and 11, and Figure 10 is a section on line Ill-l0 of Figure 9.

In a successful form of apparatus we have, as is shown in Figure 1, drawn an unknown gas mixture from a source thereof as tank 2| through conduit 22, usually a sampling hose. The gas is drawn through the conduit past flame arrestor 23 and into reaction cell 24 which is included in Wheatstone bridge circuit 26. (Figure 2.)

Examination of the gas is preferably conducted by actual burning of a portion of the gas. In the apparatus shown, platinum filament 21 is surrounded by open ended tube 28 while another platinum filament 29 is sealed in an atmosphere of air in tube 3 I Filament 21 is in one leg of the bridge while filament 29 is in the other leg. Upon a gas, having a combustible therein, contacting with filament 21, the filament being heated by current passage, the combustible burns on the filament 2'! and the reaction changes the temperature of the energized, glowing filament 2'! and thereby alters its resistance and unbalances the circuit, since the temperature of filament 29 is not affected by burning of the combustible in the bring galvanometer 32 to a zero reading, battery 35, rheostat 31 to adjust the potential applied to the filament, voltmeter 38 and switch 39.

Filaments 2! and 29 are mounted on a common base 40 which is attached by screws 4| to casing member 43 having a chamber 42 therein. Gasket 44 is provided between base 4| and member 43 so that the juncture is gas-tight. A shell 46 as of brass is placed in chambered member 43 and surrounds the filaments, being held between member 43 and gasket 44. A plurality of relatively fine holes 48 (No. '77 drill) are drilled in shell 46 so that gas from inlet 41 diffuses therethrough and enters the shell without substantially any velocity effect. Gas passed over filament 21 is drawn over that .filament practically solely because of the chimney effect of the hot vertical filament in the tube 28.

While the fine holes in shell 46 act as a flame arrestor to some extent and are effective against some'explosive mixtures, we also provide arrestors 23. These arrestors are effective alone against those relatively slow burning gas mixtures which are not comparable to hydrogen-air mixtures or acetylene-air mixtures. In a successful form we have used a block of brass having a number of effective to stop flame propagation of mixtures with the exception of certain hydrogen-air, acety' lene-air and those akin to these in having a high rate of flame propagation. The ability of this type of flame arrestor depends upon its effectiveness in reducing the temperature of the flame front. If the heat-conducting barrier is able to extract heat from the flame front, or reacting zone, sufficiently rapidly to reduce the temperature of the reacting mixture below the temperature of spontaneous ignition of the reacting mixture, the arrestor is able to prevent flame propagation beyond itself.

This type of arrestor is a practical form which is easily manufactured'and the simplest arrestor of this type is a screen. However, as previously stated, a screen is not effective against high flame rate mixtures and is subject to corrosion, clogging, and is comparatively easily ruptured or ruined as a flame arrestor by mechanically spreading the wires apart. The arrestor disclosed can also be made by inserting a long member, whose periphery is cut with a series of fine grooves, in a tube so the tube closely confines the grooves; by insertreached because of increased difficulty in forming the holes and in the hole clogging in use. No. '77 drill is at present in our opinion the optimum size. Larger holes are effective against fewer mixtures.

Now we have found that this arrestor was effective against hydrogen-air, acetylene-air and like mixtures if the advancing flame front was retarded'or its velocity decreased so that suffiicent time of contact was afforded for the heat-extractor or absorber to reduce the temperature of the flame front. Thus, if the pressure wave which accompanies the flame front, immediately before contact with the arrestor, is allowed, permitted or caused to expand, the arrestor is made effective against the otherwise unstoppable flame.

As an explanation for this we now believe, although we do not wish t b limited to this explanation, that by slowing up the pressure wave the net flame velocity is reduced. With a lower flame velocity the heat barrier which can be the arrestor has time to take up sufficient heat from the gas mixture to lower the temperature below that at which spontaneous ignition occurs and the flame is not propagated past the arrestor. Aside from the explanation, it has been established by tests on explosive mixtures of hydrogen and air that by decreasing the flame velocity, as by increasing the volume into which the flame entered immediately before the arrestor, the arrestor, which was otherwise ineffective, became effective, and the flame was not propagated past the arrestor to the gasometer or container from which the gas was drawn.

In the reaction cell shown in Figure 3, the 9 arrestor has been made effective against such mixtures as that of hydrogen and air by providing expansion chamber in member 43, by placing arrestor 23 in such a relationship to chamber 5| that the pressure wave acts as a fluid in an ejector to draw gas through inlet 41. Thus, arrestor 23 is at a right angle to passage 52 which connects the arrestor and chamber 42. A pressure wave traveling back from filament 21, upon ignition of a combustible thereon with explosive violence, through passage 52, draws gas through inlet 41 and arrestor 23. This ejector effect or draw back to prevent flame passage beyond arrestor 23 is further ensured when expansion chamber 5| is given a Venturi contour away from the point where passage 52 joins duct 53 from the arrestor. Chamber 5| has a relatively large volume compared to the volume of passage 52 and duct 53 so that a pressure wave has considerable space into which it can expand before contacting with arrestor 23. In addition, as we have stated, theejector relationship of passage 52, inlet 41 and chamber 5| and the Venturi contour of chamber 5| causes the pressure wave accompanying an advancing flame front entering chamber 5| to draw gases back through the arrestor and so reduce the net flame velocity at that point. Irrespective of any theory, the device disclosed effectively stops flame propagation through such mixtures as hydrogen and air throughout their explosive range. It is not of course necessary that gas flow actually take place from the arrestor into the passage 52 because of the draw back or ejector effect since, while that is possible, the more usual result is a reduction in net flame velocity in inlet '4'! and adjacent to and in arrestor 23.

Member 43 is cast of brass aluminum, or other suitable metal, and chamber 42 is cored therein, being afterwards machined. Expansion chamber 5|, passage 52 and duct 53 are drilled and reamed therein to communicate with each other, being afterwards sealed from the atmosphere by plugs 54. In the form shown only one expansion chamber 5| has been provided and this on the gas inlet side of the reaction cell since a flash back from the reaction cell and filament 21 to the source of gas is what is feared. In the I form shown, arrestors 23 are drilled brass plugs screwed into threaded bosses 56 integral with member 43.

Expansion chamberFlea:ibZe

In Figures 4 to 12 inclusive and 15 we have shown other devices for practising our invention and rendering an arrestor effective against a flame traveling in the direction of the arrows. These devices all have a common feature, the use of an expansion or expansible chamber to effect expansion of the advancing pressure wave and slow down the flame before it contacts with the arrestor.

In Figure 4 what is possibly the simplest form of the invention is disclosed. Arrestor BI is placed on the side of a tube 62 some distance from end 63 thereof. The end 63 is blanked off so the portion 63 between the arrestor GI and the end acts as an expansion or dead end chamber for a pressure wave traveling in the direction of the arrow. With inch 0. D. copper tubing a two inch length of inch tubing was always effective, together with arrestor 6|, in stopping hydrogen-air mixtures. Although the tubing is not flexible to any appreciable extent, the volume of tube 64 permits of expansion of the flame front and thus, in effect, provides a functionally expansible chamber, one reducing pressure wave velocity before the flame contacts the arrestor.

In Figure 5 another simple form is shown. Here a piece of rubber tubing 65 joins spaced conduits 66 and 61. Upon a flame advancing down conduit 55, as the arrow directs, the tubing expands and slows down the flame as it enters conduit 6'! to contact with arrestor 68. This is a very simple but effective device; the only disadvantage is the relatively short life of the rubber tubing.

In Figure '6 conduits II and I2 and arrestor I3 have been connected by a diaphragm cell I4 consisting of two thin flexible metal sheets which expand upon an increase in pressure therein. Normally they have a small if not a zero volume. This is an effective device, in conjunction with the arrestor against a flame advancing in the direction of the arrow along conduit 'II to cell 74 and thence into the arrestor.

The device shown in Figure 7 is an extension of that device shown in Figure 6. In this device, conduits BI and 82 and arrestor 83 are connected by a bellows 84. The bellows can be thin walled, flexible metal discs joined together and contracted to have a small, if not zero, volume so as to be expanded by the flame pressure wave but not provide a large gas pocket.

In Figures 9 and 10, a thin walled flexible metal tube 9! is flattened. This tube is inserted between conduit 92 and conduit 93 leading to arrestor 94 to provide an expansible chamber which, together with the arrestor, will stop a, hydrogenair flame traveling in the direction of the arrow.

In Figure 11 we have shown another successful device in which arrestor III is placed at an angle to conduit H2 and adiaphragm cell I I3 is placed at the end of the conduit. While conduit H2 is shown as a venturi, with arrestor III connected at throat II4 of the venturi, the device can be successfully constructed with conduit H2 comprising a length of pipe. However when a venturi is used and the arrestor is connected at the throat, passage of the flame past the arrestor connection at the throat reduces pressure in the arrestor and creates a gas flow away from the arrestor toward the flame. The net result is that the arrestor is more efiective against the flame.

In Figure 12, conduit I2I includes a sealed end I22 which forms an expansion chamber for a pressure wave advancing in the direction of the arrow to reduce the flame velocity before it passes into conduit I 23 connecting arrestor I24 to the conduit at some distance from the end. In this instance also, conduit I2I has a Venturi shape and connection of conduits I23 and I2I is made at the throat of the Venturi. The flame thus draws gases back through the arrestor and the flame velocity is slowed down materially before the flame contacts with the arrestor.

In Figure 13, casing I3I has a plurality of spaced'o-rifice plates I32 dividing the chamber into a plurality of separate communicating compartments. Casing I3I is placed between conduit I33 and conduit I34 and arrestor I35. The plates I32, having orifices therein, restrict pressure wave passage and reduce flame velocity so that arrestor I35 is effective against a flame traveling in the direction of the arrow.

Figure 14 shows another flame arrestor in which member I M is mounted in conduit I42 leading to arrestor I45. The reaction of a pressure wave advancing in the direction of the arrow on the member I4I causes the member IM to seal off substantially the opening or entrance I43 so that the flame is practically stopped, though only for a very short period. Irrespective of the mode of operation of the device, the combination is effective against the advance of a flame through hydrogen-air mixtures.

Figure 15 discloses another successful form of arrestor in which pressure relief valve I5I is placed at the end of conduit I52 to relieve pressure beyond a certain amount in conduit I52. Thus, a slit rubber tube or slit rubber diaphragm may be used or a purely mechanical device employed. Arrestor I54 is connected by conduit I55 to conduit I52 at some distance from the end thereof, in this instance at throat I55 of the venturi formed in conduit I52. Valve I5I is closed normally but under excessive pressure the valve opens and releases the pressure to the atmosphere as an infinite expansion chamber. This device when employing the Venturi utilizes the draw back through the connection I55.

We claim:

1. In combination a first conduit, means closing the end of said conduit, a flame arrestor, and a second conduit connected at one end to said arrestor and at the other end to said first conduit at a point spaced from said closing means, said second conduit being substantially at right angles to said first conduit.

2. In combination, a first conduit including a Venturi portion having a throat, means closing the end of said conduit beyond said venturi, a flame arrestor, and a second conduit connected at one end to said arrestor and at the other end to said first conduit, said second conduit being substantially at right angles to said first conduit substantially at said Venturi throat.

3. In combination, a gas passage extending generally in one direction to a blanked ofi end for said passage, a flame arrestor, and an inlet conduit having one end thereof connected to said flame arrestor and the other end thereof connected to said passage at right angles to said direction at a point spaced from said end.

4. In combination, a gas passage, a flame arrestor, means connecting said arrestor to said passage at an angle to the run of said passage, and a closed chamber connected to said passage and continuing the run of said passage beyond the point of connection of said connecting means to said passage. 7

5. In combination, a gas passage including a venturi having a throat, means closing the end of said passage to prevent gas passage, a flame arrestor, and a conduit connecting said arrestor to said venturi at said throat.

6. Means for preventing flame propagation through gas in a conduit system, said means including a first conduit, a second conduit, a flame arrestor having an inlet and outlet connected to and joining said conduits in an angular but inter communicating relation for gas passage from one conduit to the other conduit through said arrestor, and means for rendering said arrestor effective against propagation of a flame advancing down said first conduit to said second conduit,

' said means including a dead. ended expansion 

